Re: Popcount optimization using AVX512

On Sun, Apr 07, 2024 at 08:42:12PM -0400, Tom Lane wrote:
> Today's Coverity run produced this warning, which seemingly was
> triggered by one of these commits, but I can't make much sense
> of it:
>
> *** CID 1596255: Uninitialized variables (UNINIT)
> /usr/lib/gcc/x86_64-linux-gnu/10/include/avxintrin.h: 1218 in _mm256_undefined_si256()
> 1214 extern __inline __m256i __attribute__((__gnu_inline__, __always_inline__, __artificial__))
> 1215 _mm256_undefined_si256 (void)
> 1216 {
> 1217 __m256i __Y = __Y;
>>>> CID 1596255: Uninitialized variables (UNINIT)
>>>> Using uninitialized value "__Y".
> 1218 return __Y;
> 1219 }
>
> I see the same code in my local copy of avxintrin.h,
> and I quite agree that it looks like either an undefined
> value or something that properly ought to be an error.
> If we are calling this, why (and from where)?

Nothing in these commits uses this, or even uses the 256-bit registers.
avxintrin.h is included by immintrin.h, which is probably why this is
showing up. I believe you're supposed to use immintrin.h for the
intrinsics used in these commits, so I don't immediately see a great way to
avoid this. The Intel documentation for _mm256_undefined_si256() [0]
indicates that it is intended to return "undefined elements," so it seems
like the use of an uninitialized variable might be intentional.

> Anyway, we can certainly just dismiss this warning if it
> doesn't correspond to any real problem in our code.
> But I thought I'd raise the question.

That's probably the right thing to do, unless there's some action we can
take to suppress this warning.

[0] https://www.intel.com/content/www/us/en/docs/intrinsics-guide/index.html#text=_mm256_undefined_si256&ig_expand=6943

--
Nathan Bossart
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